The pressure and load conditions, for a given size of aircraft tire, are defined in particular in the Tire and Rim Association standard, commonly referred to as the TRA standard. By way of indication, the pressure to which an aircraft tire is inflated is usually at least equal to 9 bar and the corresponding applied load is such that the deflection of the tire is at least equal to 30%. The deflection of the tire is, by definition, the radial deformation or variation in radial height thereof as it makes the transition from an unladen inflated state to a statically loaded inflated state under the pressure and load conditions as defined in the TRA standard.
A high speed condition means a speed that may be as high as 360 km/h during aircraft take-off or landing phases.
Tire beads are those parts of the tire, respectively connected by two sidewalls to a tread, which provide the mechanical connection between the tire and the rim on which it is mounted or mounting rim. The assembly formed by a tire and its mounting rim is referred to as a mounted assembly.
Because a tire has a geometry exhibiting symmetry of revolution with respect to an axis of rotation, its geometry can be described in a meridian plane containing its axis of rotation. In a given meridian plane, the radial, axial and circumferential directions respectively refer to the directions perpendicular to the axis of rotation, parallel to the axis of rotation and perpendicular to the meridian plane. In what follows, the expressions “radially on the inside” and “radially on the outside” respectively mean “closer to the axis of rotation in the radial direction” and “further from the axis of rotation in the radial direction”. The expressions “axially on the inside” and “axially on the outside” respectively mean “closer to the equatorial plane in the axial direction” and “further from the equatorial plane in the axial direction”, the equatorial plane being the plane perpendicular to the axis of rotation and passing through the middle of the tread.
A radial tire comprises a reinforcement, comprising a crown reinforcement, radially on the inside of the tread, and a carcass reinforcement, radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire for an aircraft, as described for example in document EP 1 381 525, comprises at least one carcass layer.
A carcass layer is made up of reinforcing elements, or reinforcers, coated in an elastomeric material, mutually parallel and forming with the circumferential direction an angle substantially equal to 90°, i.e. comprised between 85° and 95°. The reinforcers are usually cords made up of spun textile filaments, preferably made of aliphatic polyamides and/or of aromatic polyamides.
A carcass layer is said to be turned up when, in each bead, it is wound around a circumferential reinforcing element, usually made of metal and of substantially circular meridian section, referred to as a bead wire, from the inside towards the outside of the tire to form a turnup, the end of which is radially on the outside of the radially outermost point of the bead wire. The turned-up carcass layers are generally the carcass layers closest to the interior cavity of the tire and therefore axially furthest towards the inside, in the sidewalls.
A carcass layer is said to be turned in when, in each bead, it is wound around a bead wire, from the outside towards the inside of the tire, as far as an end which is generally radially on the inside of the radially outermost point of the bead wire. The turned-in carcass layers are generally the carcass layers closest to the exterior surface of the tire and therefore those which are axially furthest out, in the sidewalls.
The mechanical connection between each tire bead and the rim is essentially achieved by two contact surfaces. A first contact surface, or contact surface under the bead wire is established, radially on the inside of the bead wire, between the radially interior face of the bead and a substantially axial rim portion, or rim seat, intended to fix the radial position of the bead when the tire is mounted and inflated. A second contact surface is established axially on the outside of the bead wire between the axially outside face of the bead and a substantially radial rim portion, or rim flange, intended to fix the axial position of the bead when the tire is mounted and inflated. The surfaces of contact between bead and flange, particularly the surface for contact under the bead wire, are zones under high pressure.
In each bead, radially on the inside of the carcass reinforcement portion radially on the inside of the bead wire, namely radially on the inside of the carcass layer which is radially innermost with respect to the bead wire, is positioned a portion of bead, referred to as the portion clamped under the bead wire, which is in contact via the radially interior face of the bead with the rim seat. This portion clamped under the bead wire dictates the forces of clamping between the bead of the tire and the rim seat. These clamping forces are dependent on the thickness of the portion clamped under the bead wire and on the mechanical properties of the material of which it is made.
The portion clamped under the bead wire comprises at least one layer called the surface layer, which is intended to come into contact with the rim seat via the axially interior face of the bead, usually made of an elastomeric material.
After curing, an elastomeric material is mechanically characterized by tensile stress/strain characteristics which are determined by tensile testing. This tensile testing is carried out by a person skilled in the art on a test specimen according to a known method, for example in accordance with international standard ISO 37, and under normal temperature (23+ or −2° C.) and moisture (50+ or −5% relative humidity) conditions as defined by international standard ISO 471. For an elastomeric material, the tensile stress measured for a 10% elongation of the test specimen is referred to as the elastic modulus M at 10% elongation and is expressed in mega pascals (MPa). The shear modulus G is defined as being equal to one third of the elastic modulus M to 10% elongation, assuming an incompressible elastomeric material characterized by a Poisson's ratio of 0.5. The shear modulus G is expressed in MPa or N/mm2. The shear stiffness K of a layer of thickness E, expressed in mm, and of unit surface area S equal to 1 mm2, made of an elastomeric material of shear modulus G, is equal to the product of the shear modulus G times the unit surface area S divided by the thickness E. The shear stiffness K is therefore expressed in N/mm.
The elastomeric material of a surface layer of a portion clamped under the bead wire usually has an elastic modulus at 10% elongation at least equal to 5 MPa and at most equal to 9 MPa.
In the case of a mounted assembly for an aircraft, a rim is dimensioned to provide a given number of take-off and landing cycles, for example 12000 cycles. The rim is inspected periodically, typically every 500 cycles. A rim is commonly withdrawn from service prematurely because of damage observed on the surfaces of contact with the beads of the tire, before it reaches the end of its theoretical life, for example midway through its life, namely after around 6000 cycles. This therefore leads to an economic loss relating to the use of the rim.
The damage is found on the rim seat in the surface for contact under the bead wire. This damage, referred to as “pitting” is surface defects of substantially elliptical shape characterized by a major axis which may be as long as 2 mm and a minor axis which may be as long as 0.5 mm, the depth of such defects potentially being as much as 0.5 mm. Defects of the maximum permissible size are referred to as critical size defects or critical defects. Beyond a critical size, defects are considered inadmissible and if they occur, the rim has to be withdrawn from service.
These defects or “pits” are the result of the known phenomenon of “wheel pitting”. “Wheel pitting” is a phenomenon of localized rim wear by abrasion. The “pits” appear on the rim seat, in the surface for contact under the bead wire which is subjected to high pressures. They are the result of the rubbing of particles, trapped between the radially interior face of the bead and the rim seat, which act as abrasives. These particles may be the result of localized damage to the painted rim coating or to external contamination. These particles, in order to be damaging, need to be sufficiently hard. These particles are pulled along by slippages of the radially interior face of the bead on the rim seat. It is the combination of high pressures and slippages that causes the localized wearing of the rim. The lengths of slippage and the rate of slippage have an impact on the localized wearing of the rim. It must be noted that slippage occurs essentially in the meridian plane, in the axial direction, and secondarily in the circumferential direction. Skewing the tire, which is characterized by a non-zero angle between the equatorial plane of the tire and the direction of travel of the tire, accentuates the phenomenon, because of the increase in pressure and slippage in the surface for contact under the bead wire.
In order to reduce the “wheel pitting” phenomenon, technical solutions based on reducing the friction between the bead and the rim seat have been envisaged, either at rim level or at tire level. As far as the rim is concerned, friction may be reduced either by lubricating the rim using a suitable product or by using a rim coating that has a low coefficient of friction. As far as the tire is concerned, friction can be reduced by selecting, for the surface layer, an elastomeric material that contains oil that is exuded from the surface layer as the bead is compressed onto the rim and which therefore lubricates the rim. Friction on the tire side can also be reduced by adding to the bead a coating material that has a low coefficient of friction. Finally, decreasing the force with which the bead wire is clamped against the rim seat can also help to reduce friction, by reducing the contact pressures between the bead and the rim seat. The foregoing technical solutions, relating respectively to the rim and to the tire, can be used alone or in combination. They all have the disadvantage of increasing the risk of the tire rotating on its rim, under the effect of too great a reduction in the forces of friction between the bead and the rim seat.